The invention relates to an environmental control system and, more particularly, to an environmental control system for use in greenhouses or the like and preferably utilizing existing power transmission lines for communication among elements of the control system.
Control of the temperature, humidity and the other measurements in a greenhouse or the like to permit the control of the environment therein can necessitate monitoring and controlling numerous sensing and control devices at various locations within the building being environmentally controlled. Due to the large number of measurements and functions that are needed to be performed, computer based or computer compatible systems have been used to centrally control the monitoring and operating functions of an environmental control system, such as in a large building.
With the advent of complex systems of environmental control a great need has evolved for monitoring systems capable of monitoring a myriad of points with respect to conditions which must be continuously observed in order to assure proper and safe operation. Similarly, alarm conditions at the points must be immediately discovered and corrected, thus requiring systems that are capable of indicating alarm conditions as well as scanning the points.
Due to the great number of remote field points that must be monitored, conventional monitoring systems utilize a control center as a receiving and sending station for monitoring the remote points which generally are scattered over great distances. Some conventional systems utilize pulse width modulation or frequency modulation to address and monitor the field points; however, these systems are extremely complex and expensive and are desirable only where extremely great distances are involved or in underdeveloped or inaccessible locations where the use of cable wires is impractical.
For environmental control in a building or complex of buildings pulse width modulation and frequency modulation systems are impractical, and systems for such application are generally based on the matrix concept as can be seen from U.S. Pat. No. 3,300,759. While the use of matrices and binary coded addresses for field points does reduce the number of wires required below the nunber of wires required for each point to be individually connected to the control central, the reduction in the number of wires is not as great as is desirable, and the number of wires required is dependent upon the number of points monitored thereby decreasing sysem flexibility. These conventional systems suffer from the disadvantages of difficult installation due to the different addresses associated with each field location and difficult system modification once the system has been installed as well as high cost of wiring. That is, each field location must be designed for a specific address thereby increasing inventory and installation time; and, if at any time additional field locations are desired to expand the system beyond the original design, additional wires are required to be installed.
Systems have been devised for reducing the number of dedicated communications wires resquired, such as shown in U.S. Pat. No. 3,613,092, but still suffer from the cost, time, and reliability disadvantages of requiring dedicated custom installed communications wiring.
Greenhouses provide weather protection for tender plants. Cultivation of the plants requires the atmosphere within the greenhouse to be maintained at a selected temperature and humidity level. Factors affecting the greenhouse atmosphere include heat gains and heat losses. For example, during long periods of sun exposure, abnormal amounts of solar energy enter the greenhouse which tends to raise the temperature.
Logical control of greenhouse environmental conditions has heretofore utilized, for example, 24 volt control systems with relays and solenoids individually wired together and strung out, or a computer based equivalent system (such as a programmable controller) with dedicated wires for communication and control strung out and wired among all control points and sensors. These systems have proved less than adequate in terms of cost, time for installation, each of maintenance, repair, and update of equipment. Additionally, communication among elements of the environmental control system has been restricted to dedicated control and communications custom wiring. Thus, expansions required a new wiring installation or modification requires a rewiring of the system.
A significant disadvantage of many prior systems involved the system reliability and maintainability, in that a breakdown in one part of the system could effectively shut down other parts of the system. Thus, to increase reliability, redundant or backup equipment was often necessitated.